Chemical contaminants have been detected by deriving the spectra of light emitted from them. As described in an article entitled "Bioimaging and Two-Dimensional Spectroscopy" published in 1990 in the Proceedings of the International Society for Optical Engineering, the spectra of fluorescence emanating from an area is derived by passing it through an interferometer that is tuned to reinforce different frequencies in the range of interest. Light passing through the interferometer is imaged onto an array of n.times.n light detectors where n is a pixel element of the detector. Each pixel element of the detector projects to an area in the field-of-view (FOV) of the measuring instrument (object space). Each pixel measures an interferogram that contains all radiation frequencies from the irradiating object within its projected object space. The electrical signals produced by the interferogram for all frequencies in the detection bandwidth are then subjected to analysis to identify the substances producing the fluorescence. The analysis involves the application of a fast Fourier transform (FFT) and several filtering operations to develop a pattern that is matched with the pattern of a known substance by a fully trained neural network. Although a Michelson interferometer could be used, the interferometer described in the article is comprised of two linear polarizers separated by birefringent crystals that can be respectively modulated or tuned by application of voltage waves. The crystals are called photoelastic modulators, PEM's. For proper operation of an interferometer, light from the emitting area being analyzed by the interferometer must be collimated to a narrow acceptance angle, or below that angle, of it's birefringement crystal. The system just described is referred to as being passive.
Analysis of an area to identify biological particles such as bacteria and viruses has been accomplished by an active system in which monochromatic polarization-modulated light, from a laser, is back-scattered by the particles and imaged onto a detector array. The light back-scattered by the biological particles is examined in the form of a Mueller matrix whose elements describe all polarization states of the back-scattered light at each pixel of the detector array. The Mueller matrix instrument is comprised of a linear polarizer and a modulated PEM positioned in the transmitted beam, a plurality of modulated PEM's and another linear polarizer positioned in the received beam. This combination of optical elements is referred to as a Mueller matrix spectrometer, MMS, and is described in an article entitled "Mid Infrared polarized Light Scattering" that was published in 1992 in the National Technical Information Series, catalog number AD-A247-359/3. Light from the last polarizer in the received back-scattered beam is imaged onto an n.times.n array of light detectors so as to produce n squared scattergrams, one per pixel in the detector array. The scattergram is processed and transformed into the Mueller elements, filtered and distributed to a neural network trained to pattern match each biological compound to its Mueller matrix characteristic.